A power supply is a particularly critical component of an active implantable medical device, because of the stringent constraints imposed by the dual needs of miniaturization and longevity. The documents U.S. Pat. No. 5,769,877 A, EP 0 719 570 A1, EP 1 647 303 A1, DE 10 2006 008 495 A1, U.S. 2008/0077010 A1, U.S. 2009/0043244 A1 and U.S. 2004/00892982 A1 describe various aspects of a power supply for a medical implantable device.
An active implantable device such as a pacemaker or an implantable defibrillator is typically provided with a battery having a nominal life of about a decade. During its usage, the battery may exhibit notable changes in the voltage between the positive and negative terminals.
Over a long term period, between the time of implantation and the end of the battery life, the average voltage decreases gradually from 3.3 to 2.4 V, a decrease of over 25% from the nominal voltage;
Over a short period, particularly as a result of sudden current peaks, the battery voltage drops due to its own internal resistance.
Concerning more specifically the latter, while the average consumption of a pacemaker is normally about 15 μA, a sudden current peak may occur from time to time, particularly when RF (radio frequency) telemetry functions are activated. This is because the power of the transmitter/receiver requires a greater current of about 5 to 10 mA.
Moreover, in the case of an implantable defibrillator, before delivery of a shock, charging the high voltage output capacitors can create a temporary current peak that can reach from 3 to 4 A. Thus, in extreme cases, the battery voltage may drop to as low as 1.2 V, a voltage drop of more than 50% within a few tens or hundreds microseconds.
These very significant long term and almost instantaneous changes in the voltage require a special power supply circuit for ensuring that the various components of the device deliver a precise and stabilized voltage. The power supply circuit used for this purpose may be of a switching converter type or a SMPS (Switching Mode Power Supply) type.
The principle of SMPS circuits is to operate cyclically, with a primary phase during which an inductor is charged with energy by the voltage delivered by the battery, and a secondary phase during which the inductor is discharged to a user circuit. Buffer capacitors at the input and output of the power supply circuit provide a certain constancy of the output voltage, whose value is regulated by varying the durations of the phases of charging and discharging and/or of the duty cycle of these durations.
Several topologies of switching converters exist, chosen according to the needs. These include so-called buck (with lowering of the voltage), boost (with increasing of the voltage), buck-boost (with lowering or increasing of the voltage, and polarity inversion), and mixed (with lowering or increasing of the voltage without polarity inversion) topologies.
On the other hand, with the same battery it is possible to combine several power supplies dedicated to supply power to the various circuits, said power supplies sharing a same inductor.
The charge and discharge phase control is generally achieved by measuring the current through the inductor. It is powered with a current that is increasing, until it reaches a predetermined value (hereinafter named “peak current”), this value being, for example, based on an error signal measured between the output voltage of the converter and the nominal voltage of the user circuit. When the peak current is reached, the load is interrupted and the inductor is discharged into the user circuit.
The charge/discharge phases can follow each other without interruption, as long as the current delivered during the discharge phase is not equal to zero. This mode of operation is referred to as continuous conduction mode or CCM, and is suitable in terms of performance when the user circuit presents a quasi-permanent relative high load. In contrast, when the discharge current in the inductor is equal to zero, a period of time may elapse between the end of the discharge phase and the start of the charging phase immediately following. This mode is referred to as discontinuous conduction mode or DCM. The DCM method is suitable in terms of performance to a user circuit with a variable load over time, for example, with a relative low load most of the time as in an AIMD.
A first drawback of known switching converters is that they generally operate satisfactorily, with a good yield, as long as the current peak does not exceed a maximum value corresponding to an operation in continuous conduction. However, for a larger current peak, the passage to a discontinuous conduction has the effect of decreasing the yield. To avoid this situation, the switching frequency is reduced to extend the duration of the phases and load the inductor with a higher energy. The counterpart is a significant increase in the peak current at the end of the charge, even when it is necessary to deliver relatively low output voltages. In other words, to increase the current to deliver to the user circuit, it is necessary to increase significantly the permissible peak current, with new constraints on the design of components so that they are able to withstand the higher peak current.
A second drawback of known switching converters is that they are generally poorly adapted to operate “on demand”, that is to say an all-or-nothing operation for a circuit that is sporadically used, such as the channels of emission/reception of an RF telemetry circuit or a circuit for delivering a defibrillation shock. In a normal operation, these circuits are not used, thus not fed, and the converter that feeds them is either shut down or put into a low power consumption mode. When they are used, the converter quickly delivers a high current in a short period of time (typically within a fraction of a millisecond). The converter therefore must be able to quickly provide the required voltage in a short period time, which is a very different situation from regulating the power supply in a continuous operation.
A third drawback is that to measure a current through the inductor, it is necessary to place in series with it a resistive element. The first consequence is the introduction of an energy loss, which reduces the overall yield. Moreover, the aging of the component over a long period (typically ten years, the usual lifetime of a cardiac implant) will result in an inaccuracy becoming larger over time, on the measurement of the peak current. The inaccuracy can reach insufficient or excessive levels of the measured peak current by comparison to the predefined value. Thus, for a maximum discharge current of about 10 mA for example, the inductor can be charged with a peak current up to 100 mA, inducing a loss of performance.
A fourth drawback is that the charge and discharge phase sequencing are performed by analog circuits. The imprecision of these analog circuits, including the dispersion of the value of the components and the drift of values during the life of the device, requires taking safety margins when designing the converter. The converter is particularly calculated on the basis of a battery voltage at the end of life, so that its operation is not optimal in early life, thus causing a loss of efficiency, unnecessarily increased consumption and oversized components.
There is, therefore, a need for an improved power supply of the switching converter type being particularly (but not limited to) suitable for active medical implants, overcoming the various constraints outlined above.